Storage phosphor screens are known in the art as screens wherein a latent X-ray image is stored when making use of a stimulable phosphor as a medium absorbing and storing radiation energy emitted by an X-ray source. Such X-rays, when having passed through an object (as e.g. a human body) provide the phosphor grains in the screen with a “latent image” which should be read out in order to make that “latent image” visible and ready for inspection by a medicine. Read-out of the X-ray image is achieved by exciting the phosphor with stimulating radiation (of longer wavelengths), thereby stimulating the phosphor to emit radiation of a shorter wavelength, which should be captured by a detector. Such a luminescent storage screen is disclosed, for example, in EP-A 0 174 875.
Holes become generated in the stimulable phosphor by incident radiant intensity, wherein these holes are stored in traps having a higher energy level, so that the latent X-ray image becomes stored in the screen, a process that seems to be very comparable with latent image formation in silver halide crystals in classical photography.
Processing however proceeds in a quite differing way: whereas in classical silver halide photography wet processing of a silver halide film material proceeds in a processing cycle throughout the steps of developing, fixing, rinsing and drying, processing of digital images requires read-out of the entire area or surface of a storage screen or panel: stimulation, pixel-by-pixel, by another radiation source, e.g. a laser, causes stimulated radiation to leave the storage panel and to be detected by a detector. Due to the stimulation radiation, the energy of the holes stored in the traps is boosted and they can fall back into lower energy levels, whereby the energy difference is radiated in the form of light quanta. The stimulable phosphor thereby emits light dependent on the energy stored in the phosphor. The light emitted as a result of this stimulation is detected and rendered visible, so that the x-ray image which was latently stored in the screen can be read out. A problem in the read-out of such screens is that the stimulable phosphor is not sufficiently transparent for the stimulating laser light. A minimum thickness of the stimulable phosphor is required to be able to achieve adequate X-ray quantum absorptions. In case however of a non-transparent, tightly compressed or sintered phosphor, the laser beam is so greatly attenuated by the phosphor that the penetration depth of the laser beam is too small. Because the energy is no longer adequate for boosting the holes to the energy level required for quantum emission, the information stored in the deeper levels cannot be read out and speed of the storage phosphor screen is reduced. Moreover as the storage phosphor particles are embedded in a binder, it is important that the said binder is made of a light-transmissive carrier material, fixing the phosphor grains. Transparency for both stimulation and stimulated radiation is thus required, in favor of speed as has been disclosed in EP-A 1 376 614. Besides its influence on speed, influence on sharpness of the captured image is another weakness: incident radiation indeed spreads increasingly with increasing penetration depth, due to scattering of the radiation beam at the phosphor grains, so that the modulation transfer function of the overall system is degraded. Providing a binderless stimulable CsBr:Eu phosphor, prepared as described in EP-A 1 203 394 and vapor-deposited in needle-shaped form as disclosed in EP-A 1 113 458 onto a carrier in a high vacuum, was forming a suitable solution for an excellent speed-to-sharpness balance.
As it was inevitable to have voids between the needles, further attempts to fill the said voids have more recently been described in EP-A's 1 316 970 , 1 347 460, and 1 349 177, wherein filling voids has been realized by measures related with application of a radiation-curable protection layer liquid, a polymeric compound and sublimated dyes respectively. Filling the voids should be considered as an alternative for needle-shaped phosphors in order to avoid destruction of the needles by compression, as well-known applied technique for powder phosphors, in order to enhance their package density in a screen. It is not excluded that powder phosphors taking advantage with respect to speed by such compression action degrade with respect to sharpness as particle boundaries between powder particles may act as scatter centers for read-out radiation.
Further measures related with support or subbing layers onto said support, taken in favor of speed and sharpness for panels with same phosphors, have been described in recent EP-A's 1 316 972, 1 316 971 and 1 341 188.
Factors particularly related with intrinsic sensitivity of the phosphors are however, to a great extent, related with incorporation (volume distribution), amount and valency of the dopant or activator element. So it is clear that incorporation of divalent or trivalent Eu into a phosphor matrix structure composed of monovalent metal compound causes deformation of the prismatic phosphor crystalline structure.
So in U.S. application 2003/0047697 the Europium signal measured at the surface of the layer is larger than the Europium-activator signal measured in the bulk of the crystal layer. The surface of the phosphor is defined therein as “1% of the total thickness of the crystal layer”. The problem is solved of “crack formation” in a layer of deposited alkali halide phosphors—by electron beam evaporation—and formation of undefined crystal face orientation in the layers, as a consequence of deformation of crystal lattice structures. A radiation image storage panel is claimed therein wherein both of the concentration of the activator component in the portion of from the bottom surface to depth of 99/100 and the concentration of the activator component in the portion of from the upper surface to depth of 1/100 satisfy, in a preferred embodiment, the condition of 0≦p/q<0.1, wherein p stands for the concentration in the portion of from the bottom surface to depth of 99/100, and q stands for the concentration in the portion of from the upper surface to depth of 1/100.
U.S. application 2003/0042429 further claims a relationship of a molar ratio of activator to mother component Ra in an optionally determined one position on the phosphor film and a molar ratio of activator element to mother component Rb in an optionally determined different position on the phosphor film, wherein said ratios are in between broad ratio values of 1:10 to 10:1, thereby providing a radiation image storage panel which shows specifically high sensitivity.
U.S. application 2003/034458 moreover claims presence as a Eu activator in a CsX containing phosphor crystal of amounts, expressed as an atomic ratio in the range from 10−4 to 10−2 in order to reach the highest sensitivity of the storage phosphor thus obtained.
U.S. application 2003/186023 in addition describes a process for preparing a radiation image storage panel comprising a support and a phosphor film comprising a stimulable europium activated cesium bromide phosphor, wherein said method comprises the steps of depositing on the support from the gas phase, europium activated cesium bromide in form of an aligned phosphor crystal layer; and heating the crystal layer in an annealing step following that vapor deposition step, in favor of providing a reproduced radiation image of a high quality with a high sensitivity.
From the considerations related with speed of storage phosphor panels given hereinbefore, it is clear that there remains a stringent demand for measures in order to further enhance sensitivity, while overcoming all probable losses in speed.